===========================
Getting Started with Mock
===========================
.. _getting-started:
.. index:: Getting Started
.. testsetup::
import os, sys
if not os.getcwd() in sys.path:
sys.path.append(os.getcwd())
import unittest
from mock import Mock, sentinel, patch
class SomeClass(object):
static_method = None
class_method = None
attribute = None
sys.modules['package'] = package = Mock(name='package')
sys.modules['package.module'] = module = package.module
sys.modules['module'] = package.module
For comprehensive examples, see the unit tests included in the full source
distribution.
Using Mock
==========
Mock Patching Methods
---------------------
``Mock`` objects can be used for:
* Patching methods
* Recording method calls on objects
.. doctest::
>>> from mock import Mock
>>> real = SomeClass()
>>> real.method = Mock()
>>> real.method(3, 4, 5, key='value')
Once the mock has been used it has methods and attributes that allow you to
make assertions about how it has been used.
`Mock` objects are callable. If they are called then the ``called`` attribute is
set to `True`.
This example tests that calling ``method`` results in a call to ``something``:
.. doctest::
>>> from mock import Mock
>>> class ProductionClass(object):
... def method(self):
... self.something()
... def something(self):
... pass
...
>>> real = ProductionClass()
>>> real.something = Mock()
>>> real.method()
>>> real.something.called
True
If you want access to the actual arguments the mock was called with, for example
to make assertions about the arguments themselves, then this information is
available.
.. doctest::
>>> real = ProductionClass()
>>> real.something = Mock()
>>> real.method()
>>> real.something.call_count
1
>>> args, kwargs = (), {}
>>> assert real.something.call_args == (args, kwargs)
>>> assert real.something.call_args_list == [(args, kwargs)]
Checking ``call_args_list`` tests how many times the mock was called, and the
arguments for each call, in a single assertion.
From 0.7.0 you can omit empty args and keyword args, which makes comparisons
less verbose:
.. doctest::
>>> real = ProductionClass()
>>> real.something = Mock()
>>> real.method()
>>> assert real.something.call_args == ()
>>> assert real.something.call_args_list == [()]
If the mock has only been called once then you can use
:meth:`Mock.assert_called_once_with`:
.. doctest::
>>> real = ProductionClass()
>>> real.something = Mock()
>>> real.method()
>>> real.something.assert_called_once_with()
If you don't care how many times an object has been called, but are just
interested in the most recent call, then you can use
:meth:`Mock.assert_called_with`:
.. doctest::
>>> mock = Mock(return_value=None)
>>> mock(1, 2, 3)
>>> mock.assert_called_with(1, 2, 3)
Mock for Method Calls on an Object
----------------------------------
.. doctest::
>>> class ProductionClass(object):
... def closer(self, something):
... something.close()
...
>>> real = ProductionClass()
>>> mock = Mock()
>>> real.closer(mock)
>>> mock.close.assert_called_with()
We don't have to do any work to provide the 'close' method on our mock.
Accessing close creates it. So, if 'close' hasn't already been called then
accessing it in the test will create it, but :meth:`Mock.assert_called_with`
will raise a failure exception.
As ``close`` is a mock object is has all the attributes from the previous
example.
Mocking Classes
---------------
A common use case is to mock out classes instantiated by your code under test.
When you patch a class, then that class is replaced with a mock. Instances
are created by *calling the class*. This means you access the "mock instance"
by looking at the return value of the mocked class.
In the example below we have a function `some_function` that instantiates `Foo`
and calls a method on it. We can mock out the class `Foo`, and configure the
behaviour of the `Foo` instance by configuring the `mock.return_value`.
.. doctest::
>>> def some_function():
... instance = module.Foo()
... return instance.method()
...
>>> with patch('module.Foo') as mock:
... instance = mock.return_value
... instance.method.return_value = 'the result'
... result = some_function()
... assert result == 'the result'
Naming your mocks
-----------------
It can be useful to give your mocks a name. The name is shown in the repr of
the mock and can be helpful when the mock appears in test failure messages. The
name is also propagated to attributes or methods of the mock:
.. doctest::
>>> mock = Mock(name='foo')
>>> mock
>>> mock.method
Limiting Available Methods
--------------------------
The disadvantage of the approach above is that *all* method access creates a
new mock. This means that you can't tell if any methods were called that
shouldn't have been. There are two ways round this. The first is by
restricting the methods available on your mock.
.. doctest::
>>> mock = Mock(spec=['close'])
>>> real.closer(mock)
>>> mock.close.assert_called_with()
>>> mock.foo
Traceback (most recent call last):
...
AttributeError: Mock object has no attribute 'foo'
If ``closer`` calls any methods on ``mock`` *other* than close, then an
``AttributeError`` will be raised.
When you use `spec` it is still possible to set arbitrary attributes on the mock
object. For a stronger form that only allows you to *set* attributes that are in
the spec you can use `spec_set` instead:
.. doctest::
>>> mock = Mock(spec=['close'])
>>> mock.foo = 3
>>> mock.foo
3
>>> mock = Mock(spec_set=['close'])
>>> mock.foo = 3
Traceback (most recent call last):
...
AttributeError: Mock object has no attribute 'foo'
Mock objects that use a class or an instance as a `spec` or `spec_set` are able
to pass `isintance` tests:
.. doctest::
>>> mock = Mock(spec=SomeClass)
>>> isinstance(mock, SomeClass)
True
>>> mock = Mock(spec_set=SomeClass())
>>> isinstance(mock, SomeClass)
True
Tracking all Method Calls
-------------------------
An alternative way to verify that only the expected methods have been accessed
is to use the ``method_calls`` attribute of the mock. This records all calls
to child attributes of the mock - and also to their children.
This is useful if you have a mock where you expect an attribute method to be
called. You could access the attribute directly, but ``method_calls`` provides
a convenient way of looking at all method calls:
.. doctest::
>>> mock = Mock()
>>> mock.method()
>>> mock.Property.method(10, x=53)
>>> mock.method_calls
[('method', (), {}), ('Property.method', (10,), {'x': 53})]
If you make an assertion about ``method_calls`` and any unexpected methods
have been called, then the assertion will fail.
Again, from 0.7.0, empty arguments and keyword arguments can be omitted for
less verbose comparisons:
.. doctest::
>>> mock = Mock()
>>> mock.method1()
>>> mock.method2()
>>> assert mock.method_calls == [('method1',), ('method2',)]
Setting Return Values and Attributes
------------------------------------
Setting the return values on a mock object is trivially easy:
.. doctest::
>>> mock = Mock()
>>> mock.return_value = 3
>>> mock()
3
Of course you can do the same for methods on the mock:
.. doctest::
>>> mock = Mock()
>>> mock.method.return_value = 3
>>> mock.method()
3
The return value can also be set in the constructor:
.. doctest::
>>> mock = Mock(return_value=3)
>>> mock()
3
If you need an attribute setting on your mock, just do it:
.. doctest::
>>> mock = Mock()
>>> mock.x = 3
>>> mock.x
3
Sometimes you want to mock up a more complex situation, like for example
``mock.connection.cursor().execute("SELECT 1")``:
.. doctest::
>>> mock = Mock()
>>> cursor = mock.connection.cursor.return_value
>>> cursor.execute.return_value = None
>>> mock.connection.cursor().execute("SELECT 1")
>>> mock.method_calls
[('connection.cursor', (), {})]
>>> cursor.method_calls
[('execute', ('SELECT 1',), {})]
Creating a Mock from an Existing Object
---------------------------------------
One problem with over use of mocking is that it couples your tests to the
implementation of your mocks rather than your real code. Suppose you have a
class that implements ``some_method``. In a test for another class, you
provide a mock of this object that *also* provides ``some_method``. If later
you refactor the first class, so that it no longer has ``some_method`` - then
your tests will continue to pass even though your code is now broken!
``Mock`` allows you to provide an object as a specification for the mock,
using the ``spec`` keyword argument. Accessing methods / attributes on the
mock that don't exist on your specification object will immediately raise an
attribute error. If you change the implementation of your specification, then
tests that use that class will start failing immediately without you having to
instantiate the class in those tests.
.. doctest::
>>> mock = Mock(spec=SomeClass)
>>> mock.old_method()
Traceback (most recent call last):
...
AttributeError: object has no attribute 'old_method'
Again, if you want a stronger form of specification that prevents the setting
of arbitrary attributes as well as the getting of them then you can use
`spec_set` instead of `spec`.
Raising exceptions with mocks
-----------------------------
A useful attribute is `side_effect`. If you set
this to an exception class or instance then the exception will be raised when
the mock is called. If you set it to a callable then it will be called whenever
the mock is called. This allows you to do things like return members of a
sequence from repeated calls:
.. doctest::
>>> mock = Mock()
>>> mock.side_effect = Exception('Boom!')
>>> mock()
Traceback (most recent call last):
...
Exception: Boom!
>>> results = [1, 2, 3]
>>> def side_effect(*args, **kwargs):
... return results.pop()
...
>>> mock.side_effect = side_effect
>>> mock(), mock(), mock()
(3, 2, 1)
Sentinel
========
``sentinel`` is a useful object for providing unique objects in your tests:
.. doctest::
>>> from mock import sentinel
>>> real = SomeClass()
>>> real.method = Mock()
>>> real.method.return_value = sentinel.return_value
>>> real.method()
Patch Decorators
================
.. note::
With `patch` it matters that you patch objects in the namespace where they
are looked up. This is normally straightforward, but for a quick guide
read :ref:`where to patch `.
A common need in tests is to patch a class attribute or a module attribute,
for example patching a builtin or patching a class in a module to test that it
is instantiated. Modules and classes are effectively global, so patching on
them has to be undone after the test or the patch will persist into other
tests and cause hard to diagnose problems.
mock provides three convenient decorators for this: `patch`, `patch.object` and
`patch.dict`. `patch` takes a single string, of the form
`package.module.Class.attribute` to specify the attribute you are patching. It
also optionally takes a value that you want the attribute (or class or
whatever) to be replaced with. 'patch.object' takes an object and the name of
the attribute you would like patched, plus optionally the value to patch it
with.
`patch.object`:
.. doctest::
>>> original = SomeClass.attribute
>>> @patch.object(SomeClass, 'attribute', sentinel.attribute)
... def test():
... assert SomeClass.attribute == sentinel.attribute
...
>>> test()
>>> assert SomeClass.attribute == original
>>> @patch('package.module.attribute', sentinel.attribute)
... def test():
... from package.module import attribute
... assert attribute is sentinel.attribute
...
>>> test()
If you are patching a module (including ``__builtin__``) then use ``patch``
instead of ``patch.object``:
.. doctest::
>>> mock = Mock()
>>> mock.return_value = sentinel.file_handle
>>> @patch('__builtin__.open', mock)
... def test():
... return open('filename', 'r')
...
>>> handle = test()
>>> mock.assert_called_with('filename', 'r')
>>> assert handle == sentinel.file_handle, "incorrect file handle returned"
The module name can be 'dotted', in the form ``package.module`` if needed:
.. doctest::
>>> @patch('package.module.ClassName.attribute', sentinel.attribute)
... def test():
... from package.module import ClassName
... assert ClassName.attribute == sentinel.attribute
...
>>> test()
If you don't want to call the decorated test function yourself, you can add
`apply` as a decorator on top, this calls it immediately.
A nice pattern is to actually decorate test methods themselves:
.. doctest::
>>> class MyTest(unittest.TestCase):
... @patch.object(SomeClass, 'attribute', sentinel.attribute)
... def test_something(self):
... self.assertEqual(SomeClass.attribute, sentinel.attribute)
...
>>> original = SomeClass.attribute
>>> MyTest('test_something').test_something()
>>> assert SomeClass.attribute == original
If you want to patch with a Mock, you can use `patch` with only one argument
(or ``patch.object`` with two arguments). The mock will be created for you and
passed into the test function / method:
.. doctest::
>>> class MyTest(unittest.TestCase):
... @patch.object(SomeClass, 'static_method')
... def test_something(self, mock_method):
... SomeClass.static_method()
... mock_method.assert_called_with()
...
>>> MyTest('test_something').test_something()
You can stack up multiple patch decorators using this pattern:
.. doctest::
>>> class MyTest(unittest.TestCase):
... @patch('package.module.ClassName1')
... @patch('package.module.ClassName2')
... def test_something(self, MockClass2, MockClass1):
... self.assertTrue(package.module.ClassName1 is MockClass1)
... self.assertTrue(package.module.ClassName2 is MockClass2)
...
>>> MyTest('test_something').test_something()
When you nest patch decorators the mocks are passed in to the decorated
function in the same order they applied (the normal *python* order that
decorators are applied). This means from the bottom up, so in the example
above the mock for `test_module.ClassName2` is passed in first.
There is also :func:`patch.dict` for setting values in a dictionary just
during a scope and restoring the dictionary to its original state when the test
ends:
.. doctest::
>>> foo = {'key': 'value'}
>>> original = foo.copy()
>>> with patch.dict(foo, {'newkey': 'newvalue'}, clear=True):
... assert foo == {'newkey': 'newvalue'}
...
>>> assert foo == original
`patch`, `patch.object` and `patch.dict` can all be used as context managers.
Where you use `patch` to create a mock for you, you can get a reference to the
mock using the "as" form of the with statement:
.. doctest::
>>> class ProductionClass(object):
... def method(self):
... pass
...
>>> with patch.object(ProductionClass, 'method') as mock_method:
... mock_method.return_value = None
... real = ProductionClass()
... real.method(1, 2, 3)
...
>>> mock_method.assert_called_with(1, 2, 3)
As an alternative `patch`, `patch.object` and `patch.dict` can be used as
class decorators. When used in this way it is the same as applying the decorator
indvidually to every method whose name starts with "test".